Closure for Dogs

Why is it acceptable to the dog’s owner that the dog pees on wheel, but it wouldn’t be acceptable if it peed on the windshield? Or in the exhaust pipe? Or on the bonnet? How does the simple fact that the wheels touch the ground change what is acceptable or unacceptable?

How will dogs achieve satisfactory pee-closure in a jetsons/maglev world where our transport doesn’t actually touch the ground? Are we likely to see infrastructure whose specific aim is to become the target of dog-pee?